This invention pertains to packaging films, and particularly to films requiring the functionality of a plurality of polymeric materials. In the most readily envisioned application, the films of this invention may be used in the packaging of dry foods such as snacks and the like. Regarding snack foods, the package should functionally serve as a barrier to intrusion of oxygen and moisture. Exclusion of oxygen from the package retards product spoilage. Exclusion of moisture prevents the product from absorbing moisture. Absorption of moisture presents two basic problems. Initially the product loses its crispness, which crispness many consumers identify as a desirable product attribute. Second, the product is more susceptible to spoilage under moist conditions, especially in the presence of oxygen; if the package admits significant quantities of oxygen and moisture combined.
While usually of lessor significance, additional factors do affect the desirability of a given film. Thus, since many snack products contain significant quantities of grease, the packaging film should provide a substantial grease barrier. Without a grease barrier, grease may penetrate through the film so that the user perceives a greasy surface on the exterior of the package. Also, since consumers identify package stiffness with product freshness and quality, a stiffer film is desirable over a functionally equivalent, but less stiff film. The film should resist scratching and scuffing of the surface and have good clarity and gloss. The film should easily accept printed graphics. Finally the film should be readily heat sealable in order to take advantage of the high speed packaging equipment commonly in use.
Currently used snack packages usually rely on cellophane, oriented polypropylene (OPP), or oriented polyester in the package structure. Typical structures include single layer cellophane, double layer cellophane, and cellophane combined with a layer of glassine, OPP or a sealant such as ethylene vinyl acetate (EVA). Another known structure is OPP and a sealant.
The above films do provide certain desirable features. Cellophane is stiff and gives a good freshness perception. OPP provides good moisture barrier and is resistant to flex cracking. Glassine is a low-cost, stiff layer providing excellent opacity.
For oxygen barrier it is known to provide a layer of saran at a convenient layer interface.
While the individual layers provide certain benefits, existing snack films represent a compromise between performance and cost, and construction of a highly acceptable package from these materials could be very expensive. First would be the production of a multiplicity of individual layers, and then lamination of them together to give a highly desirable, but expensive film. A problem with economics is that such layers as OPP and glassine are not readily adapted for in line web production and lamination.
Further to the problems of manufacturing such films, it is highly desirable that the film surface which is predetermined to be on the outside of the package should have good slip characteristics for machining purposes. But slip additives tend to resist adhesion of printing inks which are required for graphics and advertising purposes. For example, a structure of OPP and EVA can be machined easily if a slip agent is incorporated into the OPP layer; however, ink adhesion is poor. Ink adhesion is satisfactory if the slip agent is omitted, but then the film doesn't operate very well on conventional packaging machinery. If the ink is, rather, printed before lamination onto the surface of the OPP which will be laminated to the EVA, then residual ink volatiles may permeate to both the OPP and the EVA film surfaces of the package, risking contamination of the product at the film surface on the EVA side of the film.
Further to cost effective production of the packaging film, it is desirable that all layers of the film be susceptible to economic in-line production and combination.
Finally it is submitted that the surface characteristics of the film are critical not only to functional performance, but also to perceived value in terms of the contained product. Thus such characteristics as gloss, scuff resistance and heat resistance are important in the film surface.